Histological and ultrastructural characterization of the alimentary system of solifuges (Arachnida, Solifugae)

Description of the larval and adult hindgut tract of the common spider crab Maja brachydactyla Balss, 1922 (Brachyura, Decapoda, Malacostraca)

Arthropods are the most diversified animals on Earth. The morphology of the digestive system has been widely studied in insects; however, crustaceans have received comparatively little attention. This study describes the hindgut tract of the common spider crab Maja brachydactyla Balss, 1922, in larvae and adults using dissection, light and electron microscopical analyses. The hindgut tract maintains a similar general shape in larvae and adults. Major differences among stages are found in the morphology of epithelial cells and microspines, the thickness of the cuticle and connective-like tissue, and the presence of rosette glands (only in adults). Here, we provide the description of the sub-cellular structure of the folds, epithelium (conformed by tendon cells), musculature, and microspines of the hindgut of larvae and adults of M. brachydactyla. The morphological features of the hindgut of M. brachydactyla are compared with those of other arthropods (Insecta, Myriapoda and Arachnida). Our results suggest that the morphology of the hindgut is associated mainly with transport of faeces. In adults, the hindgut may also exert an osmoregulatory function, as described in other arthropods. At difference from holometabolous insets, the hindgut of M. brachydactyla (Decapoda) does not undergo a true metamorphic change during development, but major changes observed between larval and adult stages might respond to the different body size between life stages.

Histological characterization of the gastrointestinal tract of the adult horseshoe crab (Limulus polyphemus) with special reference to the stomach

The American horseshoe crab (Limulus polyphemus) is one of four extant species in the Order Xiphosura, subphylum Chelicerata, and are evolutionarily more closely related to scorpions and spiders, than crabs. The basic structure, function, and physiology of these invertebrates and their internal organs are not well documented in the literature. In this study, the gastrointestinal system, with a focus on the stomach, of adult L. polyphemus were assessed by gross and histologic methods to further characterize the pyloric valve, the lining of the ventricular lumen, and the muscular tunics of the stomach. Determination of normal anatomical structure of this organ system, along with characterization of the esophagus and intestinal tract, will set a standard against which tissue abnormalities, such as those seen with disease or pathology were to arise, would allow for better interpretation.

Differentiation and function of the ovarian somatic cells in the pseudoscorpion, Chelifer cancroides (Linnaeus, 1761) (Chelicerata: Arachnida: Pseudoscorpionida)

Pseudoscorpion females carry fertilized eggs and embryos in specialized brood sacs, where embryos are fed with a nutritive fluid produced and secreted by somatic ovarian cells. We used various microscopic techniques to analyze the organization of the somatic cells in the ovary of a pseudoscorpion, Chelifer cancroides. In young specimens, the ovary is a cylindrical mass of internally located germline cells (oogonia and early previtellogenic oocytes) and two types of somatic cells: the epithelial cells of the ovarian wall and the internal interstitial cells. In subsequent stages of the ovary development, the oocytes grow and protrude from the ovary into the hemocoel (opisthosomal cavity). At the same time the interstitial cells differentiate into the follicular cells that directly cover the oocyte surface, whereas some epithelial cells of the ovarian wall form the oocyte stalks - tubular structures that connect the oocytes with the ovarian tube. The follicular cells do not seem to participate in oogenesis. In contrast, the cells of the stalk presumably have a dual function. During ovulation the stalk cells appear to contribute to the formation of the external egg envelope (chorion), while in the post-ovulatory phase of ovary function they cooperate with the other cells of the ovarian wall in the production of the nutritive fluid for the developing embryos.

Controls on gut phosphatisation: the trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)

Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagerstätten. However, the reasons for their fossilisation and their biological implications remain to be thoroughly explored. This is particularly true with arthropods – typically the most diverse fossilised organisms in Cambrian ecosystems – where digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on their biology. Here we describe the phosphatised digestive structures of three trilobite species from the Cambrian Weeks Formation Lagerstätte (Utah). Their exquisite, three-dimensional preservation reveals unique details on trilobite internal anatomy, such as the position of the mouth and the absence of a differentiated crop. In addition, the presence of paired pygidial organs of an unknown function is reported for the first time. This exceptional material enables exploration of the relationships between gut phosphatisation and the biology of organisms. Indeed, soft-tissue preservation is unusual in these fossils as it is restricted to the digestive structures, which indicates that the gut played a central role in its own phosphatisation. We hypothesize that the gut provided a microenvironment where special conditions could develop and harboured a source of phosphorus. The fact that gut phosphatization has almost exclusively been observed in arthropods could be explained by their uncommon ability to store ions (including phosphorous) in their digestive tissues. However, in some specimens from the Weeks Formation, the phosphatisation extends to the entire digestive system, suggesting that trilobites might have had some biological particularities not observed in modern arthropods. We speculate that one of them might have been an increased capacity for ion storage in the gut tissues, related to the moulting of their heavily-mineralised carapace.